* src/signalutils.{cc,h} (AutoConnection): New utility class that breaks
the previous connection on assignment, and also remembers the blocked state
of the connection permanently.

(ScopedBlock): Use Util::AutoConnection instead of SigC::Connection.

* src/filetree.{cc,h}: Use AutoConnection to manage the connection handles
of the currently selected buffer.

(on_selection_changed): Thanks to AutoConnection, we can now extinguish
a fair bit of error-prone code.